This invention relates to a new method and device for removing NO.sub.x from gaseous material, e.g., from exhaust gas streams.
The recent emphasis on ecological and environmental concerns, especially air pollution, acid rain, photochemical smog, etc., has engendered a wide variety of proposed methods for removing NO.sub.x, especially NO, from gas streams.
Certain proposed techniques involve a great deal of capital outlay and require major consumption of additives, scrubbers, etc. For example, U.S. Pat. No. 3,894,141 proposes a reaction with a liquid hydrocarbon; U.S. Pat. No. 4,405,587 proposes very high temperature burning with a hydrocarbon; U.S.P. 4,448,899 proposes reaction with an iron chelate; and U.S. Pat. No. 3,262,751 reacts NO with a conjugated diolefin. Other methods utilize reactions with nitriles (U.S. Pat. No. 4,080,425), organic N-compounds (e.g., amines or amides) (DE No. 33 24 668) or pyridine (No. J57190638). Application of these reactions imposes organic pollutant disposal problems along with the attendant problems of toxicity and malodorous environments. In addition, they require the presence of oxygen and are relatively expensive.
Other systems are based on urea reactions. For example, U.S. Pat. No. 4,119,702 uses a combination of urea and an oxidizing agent which decomposes it, e.g., ozone, nitric acid, inter alia; U.S. Pat. No. 4,325,924 utilizes urea in a high temperature reducing atmosphere; and U.S. Pat. No. 3,900,554 (the thermodenox system) utilizes a combination of ammonia and oxygen to react with nitric oxide. All of these methods must deal with the problem of the odor of ammonia and its disposal. All require oxygen or other oxidizing agents. These methods also suffer from the drawback of requiring controlled environments which make them difficult to use in mobile vehicles or smaller stationary devices.
Japanese Publication No. J55051-420 does not relate to the removal of nitric oxide from gaseous systems, at least as reported in Derwent Abstract 38871C/22. It utilizes halocyanuric acid to remove malodorous fumes, e.g., mercaptans, sulfides, disulfides, ammonia or amines from gases by contact therewith followed by contact with activated carbon. Temperatures are reported as less than 80.degree. C.; classical acid/base interactions appear to be involved (not pyrolysis decomposition products of the halocyanuric acid).
Back et al, Can. J. Chem. 46, 531 (1968), discusses the effect of NO on the photolysis of HNCO, the decomposition product of cyanuric acid. An increase of nitrogen concentration in the presence of large amounts of nitric oxide (torr levels) was observed utilizing a medium pressure mercury lamp for photolysis of HNCO. High temperature reactions were neither addressed nor involved; similarly, the effect, if any, of HNCO under any conditions on low amounts of NO (e.g., in the &lt;torr to ppm range) was also not addressed. In fact, the increased concentration of nitrogen was associated by the authors with high NO levels. Their theorized reactions explaining the results would be important only at high NO levels.
Furthermore, use of cyanuric acid as a source of isocyanic acid (HNCO) for purposes of studying various properties of the latter or its subsequent degradation products is also known. See, e.g., Okabe, J. Chem. Phys., 53, 3507 (1970) and Perry, J. Chem. Phys., 82, 5485 (1985).
J.P. No. 54-28771 discloses the addition of relatively large particles (0.1-10 mm, preferably 0.5-5.0 mm) of cyanuric acid at temperatures generically disclosed as 600.degree.-1500.degree. C., but preferably at high temperatures of 1200.degree.-1300.degree. C., for removal of NO.sub.x from exhaust gas. The theory of operation disclosed in this publication appears to involve a reaction occurring on the surface of the particle which leads to the requirements of the high particle size and high temperature. It is explicitly stated in the publication that, "If the diameter of the granule is too small, the efficiency goes down." There is no suggestion in this publication that the active species effecting the treatment of the exhaust gas is itself gaseous and certainly no suggestion that the gaseous species is HNCO. As a result, the conditions disclosed in this reference lead away from those which are most applicable to a reaction of NO.sub.x with gaseous HNCO. Consequently, the process of this reference is believed not to have been used on a technical scale.
As a result, there continues to be a need for a simple, relatively inexpensive, non-polluting, non-toxic, non-malodorous and regenerable system, method and device for removing nitric oxide from gas streams.